The present invention relates to a fuel injection system for an internal combustion engine, for providing an injection event comprising a first stage and a second stage via a single nozzle which is connected by its inlet port to a source of variable fuel pressure, said nozzle including a needle valve for performing the first stage of injection, and a poppet valve for performing the second stage of injection.
The present invention concerns fuel injection systems of internal combustion engines, in particular systems for injection of fuel directly into combustion cylinders of compression ignition engines.
Compression ignition or diesel engines will, according to most forecasts, remain the dominant mechanical power source for transportation, construction and other machinery in the foreseeable future. However, depletion of reserves and rising cost of crude oil that at the present time remains practically the only source of fuel for diesel engines, initiate efforts aimed at finding alternative fuels suitable for diesel engines. One particularly promising fuel, both in terms of its environmental characteristics and suitability for efficient diesel operation, is dimethyl ether, or DME. Chemical and thermodynamic properties of DME significantly differ from that of traditional diesel fuel though, requiring optimization of fuel injection system to ensure efficient, operation of same and thus of engine as a whole.
Among the most important differences between DME and traditional diesel fuel oil are significantly lower calorific value and density of the former and vastly greater sooting tendency of the latter. The lower calorific value and density of DME, combined, make it necessary to inject almost twice the volumetric amount compared to diesel oil in order to obtain the same engine power. The difficulties of creating high DME injection pressures, arising from its much poorer lubricity, lower viscosity and greater compressibility, make it necessary to utilize nozzles with very large flow areas to achieve the high flow rates and injected volumes. This creates certain difficulties for conventional diesel nozzle designs featuring a needle valve controlling flow to spray orifices, arising from too large orifice number and diameter required. On the other hand, the much lower sooting tendency of DME presents the advantage of being able to utilize the other type of nozzle where large flows are easily attainable but which cannot be used in contemporary diesel oil-fueled engines due to in that case unacceptably high soot emissions.
One such nozzle type is a poppet nozzle with the poppet opening outward against the forces of a return spring and backpressure in the combustion chamber of the engine. The use of nozzles of this type had been discontinued in the diesel engine industry long time ago, although later on there have been attempts, so far not reaching commercial application, to revive the concept, driven by either the relative simplicity of the design or its suitability for being adapted for two-stage operation. An example of a more recent development is disclosed in the U.S. Pat. No. 6,513,487 B1. In that design, a poppet nozzle's not-so-favourable for diesel combustion property of very quick opening of a large flow area with fuel sprayed in the form of a hollow cone, is attempted to be eliminated through the use of a cylindrical poppet guide extending all the way down to the main tapered seat of the poppet, such that the bottom edge of the nozzle body guide surface provides a spool-like area control for the spray orifices formed in the poppet guide in the vicinity of the poppet seat. This solution allows the use of spray holes of axially elongated shape and/or multiple rows of holes having different size/direction for control of initial combustion rates etc., as disclosed in the document. Operation on DME, thanks to low sooting quality of the fuel, is likely to be forgiving to this design's propensity to fuel splashing and fuel film formation on external nozzle surfaces, but the exposure of the guide, which has to be relatively closely matched for effective orifice edge control, to the hot and contaminating environment of the engine combustion gases can severely undermine reliability of function. Therefore, the more traditional designs of the poppet valves with waisted stem portion adjacent to the poppet seat, have better prospects in terms of reliability.
As indicated by research and experience, the DME diesel combustion process can, in terms of NOx-soot-BSFC tradeoffs, benefit from careful control of the injection rate in the beginning of fuel injection. Even pilot injections can be beneficial in certain conditions. Achieving that can however be complicated by the fact that the maximum flow area of the nozzle has to be large due to reasons explained above, and is certainly difficult in case of a poppet nozzle which normally tends to open a large area quickly in the beginning of injection. The present invention addresses this difficulty by providing simple and effective means of accurately controlling pilot injections and initial rate of injection in a poppet type of nozzle.
A prior art injector system with certain similarity is described in EP 0980475B1. That system is designed for operating with two fuels simultaneously, one of the fuels being a pilot fuel for igniting the other, main fuel such as natural gas. The injector is consequently a complex apparatus with multiple inlet/outlet ports and is additionally complicated by separate valves for relieving the pressure of actuating fluid used to open the nozzle etc.
It is desirable to provide a fuel injection system with relatively large maximum nozzle flow area, such as that required for injecting relatively low-density and low specific heat fuels, for instance DME, which is capable of producing pilot injections and achieving rate shaping in the beginning of injection with good accuracy and fuel spray quality, it is desirable to provide a double-stage nozzle with a needle valve capable of opening spray orifices with relatively small flow area during a first stage of fuel injection, designed for delivering fuel at a slower and accurately controlled rate, and with a poppet valve capable of opening relatively large flow area and achieving relatively high injection rate when moving outwards toward the engine combustion chamber in a second stage of fuel injection.
It is desirable to provide a fuel pressure-controlled double-stage nozzle in which the activation of the first and second stages of injection can be selected by controlling the pressure at the inlet of the nozzle, and in which the operation of the needle valve can also be controlled by the movement of the poppet valve for achieving better injection characteristics.
The fuel injection system according to an aspect of present invention contains a source of variable fuel pressure to which an inlet port of a nozzle is connected. The nozzle incorporates a poppet valve which has a poppet and is biased by a poppet return spring towards its closed position, in which the poppet abuts against a poppet seat formed on the nozzle and closes a flow area between them, through which fuel under pressure can otherwise be injected out of the nozzle and into engine's combustion chamber. The area of the poppet valve enclosed within the diameter of the poppet seat is exposed to the pressure in the inlet port which can, upon rising to a predetermined level defined by the seat diameter, poppet return spring preload and backpressure outside the nozzle, open the nozzle by moving the poppet valve toward the combustion chamber of the engine against the force of the poppet return spring and of the pressure in the combustion chamber.
There is a bore in the poppet valve which extends axially from the top of the valve and terminates by at least one injection orifice in the bottom part of the poppet valve, the injection orifice opening out to the combustion chamber of the engine. A needle valve is installed in this bore, with a cylindrical guide in its upper portion producing a precision-matched sliding fit with the bore. The needle valve also has a seat formed on its bottom portion which can engage with the bottom of the bore to close the fluid communication between the bore and the injection orifice. The volume of the bore confined between the needle valve seat and the needle guide is always connected to the inlet port of the nozzle. A spring cap fitted at the top of the poppet valve, the guide of the needle valve and the bore form a needle spring chamber in which a needle return spring is installed that biases the needle to close the injection orifice, in use, the spring cap does not allow fluid communication between the needle spring chamber and a poppet spring chamber.
The poppet valve and the nozzle body form a precision-matched poppet guide in which the poppet valve can slide up and down to close and open the nozzle. A return channel is provided in the nozzle body which opens up onto the poppet guide, either directly or via an annular return groove. An outlet control orifice for connection of the needle spring chamber to the return channel is provided in the poppet valve such that the positions of the needle valve and the poppet valve can control the flow area of this outlet control orifice. Similarly, there is a supply channel in the nozzle body, which is connected to the inlet port and which, on the other end, opens up onto the poppet guide, either directly or via an annular supply groove. An inlet control orifice for connection of the needle spring chamber to the supply channel is provided in the poppet valve such that the position of the poppet valve can control the flow area of this inlet control orifice. The clearance in the poppet guide is sufficiently small to minimize leakage of pressurised fuel along the guide and to ensure necessary reduction of flow in control orifices upon their overlapping with the edges of the channels or annular grooves in the nozzle body.
In the closed position of the nozzle, the needle spring chamber is connected by the outlet control orifice to the return channel and is disconnected from the inlet control orifice because of the misalignment between the inlet control orifice and the supply channel, such that the pressure in the needle spring chamber equals the return port pressure. The opening pressure of the needle valve is set by an appropriate combination of the needle return spring preload and the size of the needle differential area (defined by the needle guide diameter and the needle seat diameter) to be lower than the opening pressure of the poppet valve. When the pressure in the inlet port rises for the first stage of the injection process to begin, the needle valve opens allowing fuel to be injected through relatively small injection orifices in the poppet.
When injection at a higher rate is required, the pressure in the inlet port is increased further and above the opening pressure of the poppet valve, which then moves downward and opens a large flow area between the poppet and its seat allowing fuel to escape from the poppet pressure chamber out to the combustion chamber, thereby commencing a second stage of the injection. During this downward movement of the poppet valve, the outlet control orifice becomes overlapped by the edge of the return channel or groove, closing the flow path from the needle spring chamber to the return port. Further opening of the poppet valve aligns the inlet control orifice with the supply channel so that the fuel under pressure flows into the needle spring chamber and assists the needle return spring in closing the needle valve. Thus the needle valve can be closed quickly upon opening of the poppet valve.
To end the injection, the pressure in the inlet port is reduced below a level that can keep the poppet valve open against the force of the poppet return spring and the backpressure in the combustion chamber. The poppet valve then moves upward and closes whilst the needle valve remains closed by the force of the needle return spring.
By these means, a fuel injection system with a double-stage nozzle is provided that allows for accurate control of small fuel deliveries necessary for idle and low load operation of the engine, for effective rate-shaping of injection and for achieving high flow rates of injection of large fuel quantities, at the same time ensuring low control leakages and a relatively simple design. Additionally, the system achieves quick end of injection.
The number, direction and the total flow area of the injection orifices, on one hand, and the poppet nozzle settings, on the other hand, can be optimised independently to ensure the best fuel distribution and rate of injection required in different engine operating conditions, typically low load and speed operation as opposed to high-load operation. The selection of either needle or poppet valve to be open, and the duration of their opening, is made through controlling the fuel pressure in the inlet port of the nozzle, which can be carried out in a number of ways that are known in the art and that will be reviewed in more detail in the following sections of the description.